1. Field of the Invention
This invention pertains generally to implantable catheters, and more particularly to self-clearing catheters.
2. Description of Related Art
One in every 500 newborns is afflicted by hydrocephalus, a condition characterized by an abnormal accumulation of cerebrospinal fluid (CSF) due to an imbalance between cerebrospinal fluid production and absorption. In addition, hydrocephalus may also be acquired later in life due to incidents such as head trauma or tumor development. Fortunately, treatments have greatly increased the probability for hydrocephalic patients to achieve normal intelligence and to lead a normal life.
Implanted medical catheters are now an integral part of clinical care. Over 25,000 shunt operations are completed each year in the U.S. alone. Patients who receive implanted shunts are dependent on the device functioning properly.
However, many chronically implanted catheter systems are often plagued with reduced performance due to the prolonged accumulation of biological debris. For the neurological disorder of hydrocephalus, obstruction of the shunt tubing that diverts cerebrospinal fluid (CSF) from the brain is one of the most commonly occurring complications, which can result in a catastrophic shunt failure that could inflict serious bodily harm to the patient.
A malfunctioning (or obstructed) shunt can be a life-threatening condition. On average, 85% of people with shunts have at least two shunt-revision surgeries in their lifetime. A minority of patients are plagued with recurrent shunt obstructions and may undergo over 100 shunt revisions. Each successive shunt revision may cause brain injury and increases the risk of shunt infection. Not only are shunt-replacement surgeries a cause of morbidity and stress for patients and families, but they also impose economic burdens on the patient and society.
FIG. 1 illustrates a system 10 having shunt tubing to divert cerebrospinal fluid (CSF) from brain 14. System 10 generally comprises a ventricular catheter 20 positioned within affected ventricle 14. The ventricular catheter 20 is coupled to tubing 24 via a valve 22. The tubing 24 runs the length of the torso to drain within the low abdomen 28. Shunt obstructions can occur at various locations around the implanted system 10. Certain locations, such as the distal end of the catheter 26, the valve 22, and the ventricular catheter 20, however, are most often implicated.
Although recent improvements in valve designs and antimicrobial coatings have reduced the occurrence of obstructions at the valve 22 and the distal end of the shunt 26, methods to prevent the ventricular catheter 20 obstruction still remain elusive.
There are multiple recognized causes of the obstruction at the ventricular end of the shunt system, the primary ones being summarized below:
1. Gradual accumulation of cells in flow pores: FIG. 2 illustrates a currently available ventricular shunt system using a ventricular catheter 20 having a plurality of intake pores 30 at end 32 of the catheter. As seen further in the cross-sectional view of FIG. 3, cells 36 may accumulate in and around intake pore 30 to partially and then fully occlude the opening 30. Hydrocephalus often results in an increased numbers of cells in the patients' CSF (pleocytosis). Similar pathology can be found in patients with chronic meningitis, and it is thought to be one of the main causes of shorter shunt half-lives for these patients. Indeed, non-pleocytic CSF does have lower cellular concentration. However, even in hydrocephalus patients without pleocytosis, the shunt system moves hundreds of millions of cells to through its catheter pores in its lifetime such that the cellular occlusion is almost a certain eventuality. As such, the cellular occlusion is thought to be one of the main causes of shunt malfunction in hydrocephalus patients.
2. Ventricular collapse due to excess drainage: Ventricular collapse following shunting procedures has been associated with shunt obstruction. A primary focus of valve 22 design has been to limit excessive drainage and therefore prevent the collapse of the ventricles 14. Although the incorporation of valves with an adjustable opening differential pressure that control the rate of CSF flow have been touted to maintain an ideal ventricular size and intracranial pressure, these goals have not been consistently achieved clinically. The mechanism by which obstruction occurs with ventricular collapse is related to the direct apposition of ependymal and/or choroid plexus tissue with the ventricular catheter tip. The close proximity to the ependymal wall provides abundant supply of cells to accumulate on the ventricular catheter pores. Despite advances in valve technologies, ventricular collapse continues to be an increased risk of shunt obstruction.
3. Choroid plexus tissue migration and ingrowth: Choroid plexus tissue migration occurs in situations where the catheter flow holes are in close proximity with the choroid. The suctioning effect, which is inherent in many shunt designs, can draw the choroid tissue directly into the catheter pores. FIG. 2 shows a prior art flanged catheter tip 20b having a plurality of radial flanges 34 at the site of the apertures 30, which was introduced with the goal of preventing the choroid tissue from accessing the flow holes 30. The clinical experience with this design, however, has been mixed. Proximal catheter obstructions have not been prevented and the reason is not clear. Assuming choroid tissue was indeed impeded, cells freely floating in CSF presumably led to the obstruction. Some studies have suggested that optimal placement of the catheter tip is at a location that is out of the reach of the choroid plexus. Anatomically, this placement goal is very difficult to achieve with current catheter designs. With better catheter designs and judicious use of endoscopy, this placement goal may be achieved.
Cellular occlusion is thought to be one of the main causes of failure for the chronically implanted catheters in hydrocephalus patients. Pathological studies have shown that the cellular composition of catheter obstruction consists mainly of: red blood cells, inflammatory cells, and proteins. Though not commonly present in CSF, red blood cells are often introduced into the CSF due to hemorrhaging that occurs during shunt placement surgery. Thus, in a long-term scale, red blood cells do not have substantial impact on obstruction formation. Red blood cells, however, are susceptible to coagulation, which could result in larger body of mass that may ultimately obstruct catheter pores. Inflammatory cells and other associated proteins are identified as the main components of the cellular accumulation. With introduction of foreign material (catheter) into the brain, the white blood cells (leukocytes) undergo a delayed hypersensitivity reaction that leads to increased cellular adhesion. A proteinaceous layer that forms on the surface of the catheter facilitates this process by allowing formation of arginine-glycine-glutamic acid (RGD) receptor-ligand complex that recruits more leukocytes to the silicone surface.
Biomedical approaches, such as use of a silicone elastomer, grafting, hydrophilic, lubricious hydrogel onto the silicone surface, and drug-eluting catheters, have yet to display consistent performance over long period of time.
Microfabricated self-clearing catheters have previously been used (U.S. Patent No. 2008/0281250) to incorporate microfabricated magnetic actuators into conventional ventricular catheters to combat cellular occlusion. The magnetic microactuator produces out-of-plane movements to allow for disruption or prevention of biological accumulation at the flow pores. However, one major caveat of this approach is that the device must reside within the catheter pore, and thus is prone to cause additional hindrance to the normal flow of CSF. Because this torsional-microactuator-design maintains a horizontal rest position, it greatly reduces the open pore area at rest and occludes much of the catheter pore while in this state.
Accordingly, an object of the present invention is a system and fabrication process for magnetic microactuators that stay clear of the catheter pore while at rest.